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1. Field of the Invention
The present invention relates generally to a mold apparatus and method for manufacturing panels. More particularly, this invention relates to a method for manufacturing panels having, as desired, one or more areas of reduced material comprising a facing sheet and cell forming walls extending in roughly perpendicular fashion from said facing sheet, wherein said cell forming walls are integrally formed with and attached to said facing sheet. More particularly still, the present invention relates to a method of molding panels from thermoplastic resins and other moldable materials which requires significantly less energy than other known molding methods. More particularly still, the present invention relates to a method of molding panels from thermoplastic resins and other moldable materials wherein said panels have one or more sections which exhibit characteristics of a solid panel, that is, portions of said panels which have roughly continuous outer surfaces.
2. Description of Related Art
Panels constructed of various materials, and embodying many different sizes and shapes, have been known in the art and used in a multitude of applications for some time. Although such applications are too numerous to name, some examples include the use of large-scale panels made of wood, gypsum, metal and/or fiberglass in connection with the construction of buildings and other structures. Additionally, panels of various sizes have routinely been used, individually and in combination with one another, as load-bearing elements.
In most applications, it is beneficial for panels to exhibit desirable strength characteristics, while remaining relatively light. In this regard, attempts have been made to utilize thermoplastic resins and other synthetic materials to replace materials like metal, wood and concrete to construct such panels. However, to date, it has been difficult to manufacture panels from thermoplastic resins and other moldable materials having the desired strength and weight characteristics in a cost effective manner. This is particularly true in the case of large-size panels.
While it may be beneficial to manufacture solid panels, it is generally known that use of cellular structure in panels can greatly reduce material requirements while maintaining, or in some cases actually increasing, strength characteristics of said panels. In most instances, such material reduction results in significant cost savings. In the case of panels molded from thermoplastic resins and other moldable synthetic materials, the use of cellular structure to reduce material requirements can have a dramatic effect on the ultimate cost of such molded panels.
Panels exhibiting cellular structure, in general, and hexagonal shapes or honeycombs, in particular, have been known in the art for some time. Similarly, molding apparatuses and methods of manufacturing panels containing cellular structure are also generally well known in the art.
U.S. Pat. No. 2,892,339 to Flower et al, discloses panels having open faced cellular structure which are constructed of gypsum plaster or other moldable composition, as well as a method of manufacturing same. The panels described in the ""339 patent include at least one facing sheet having integral and homogenous cell forming walls disposed with their axes at a right-angle to said facing sheet. The ""339 patent further describes a method of manufacturing such panels comprising a facing-mold table for containing a volume of plaster or other moldable material, a series of deformable plugs or offsets for molding cell walling in one with a facing layer of plaster or other moldable material, a vertically movable framing on which such plugs are mounted in spaced relation, means for immersing said plugs in plaster or other moldable material of the facing-mold to form the whole of the cell walling for a panel simultaneously with the facing, and means for contracting the plugs for their withdrawal from the plastic or other moldable material.
U.S. Pat. No. 3,509,005 to Hartig discloses a method of applying molten thermoplastic resin into a pattern disposed on the surface of a rotating roll to form a web of ribs integrally molded to a continuous film.
U.S. Pat. No. 3,617,416 to Kromrey describes a structure consisting of fiber-reinforced cells, such as honeycombs, which are integrally molded to a substrate material, as well as a method of manufacturing said structure. The method described in the ""416 patent utilizes dies formed as continuous individual strips of cells having a desired end configuration. Said die strips are placed side by side and interfitted to form a mold for a honeycomb structure. Fibers or fabric pre-impregnated with curable resin are wound between the dies to form cellular walls of a honeycomb structure, as well as a substrate layer. The entire structure is then subjected to conditions of elevated temperature and pressure to effect a cure.
U.S. Pat. No. 5,776,582 to Needham discloses loadbearing structures having interlockable edges. The ""582 patent addresses a method of molding panels having cellular structure out of thermoplastic resins and other materials. The ""582 patent briefly describes a pan-like lower mold member into which thermoplastic material and reinforcing fiber is placed. Thereafter, an upper mold member having positive plug extensions are combined with the lower mold member, and heat and pressure is applied. The positive plug extension is used to deform the resin and reinforcing material to the desired configuration.
Similarly, U.S. Pat. No. 5,888,612 to Needham, et al, also describes a method of molding panels containing cellular structure. A lower mold member having an ordinary smooth interior surface is first filled with thermoplastic material. An upper mold member is configured with positive molding dies. Said upper and lower mold members are combined, and the dies of said upper mold member are used to displace said thermoplastic material. Thereafter, the apparatus is subjected to conventional heat and pressure to form the desired panel having cellular structure.
One significant problem encountered when molding panels having cellular structure under the prior art relates to uniform placement of thermoplastic resin or other moldable material which form cell walls. Under the prior art, thermoplastic resin or other moldable material is loaded into a lower mold member which is typically in the form of a pan. Thereafter, an upper mold member having positive standoffs, spaced apart in a desired pattern, is forced into said lower mold member. In order to create the desired cell-forming walls, the thermoplastic resin or other moldable material must be displaced by said upper mold member in order to fill voids between said positive standoffs. Unless such resin or other material is uniformly and adequately distributed between said positive standoffs, pits or voids will often develop in the cell-forming walls. Because such thermoplastic resin or other moldable material must be mechanically displaced between the positive standoff elements of the upper mold member, the prior art methods of molding have significantly greater energy requirements than the molding method of the present invention disclosed herein.
Additionally, it is often beneficial to manufacture panels of thermoplastic resin which have one or more generally solid sections, that is, sections exhibiting roughly the same characteristics as a solid panel, such as continuous outer surfaces. However, with existing molding methods described in the prior art, it is generally not possible to mold large-size panels or other items having one or more portions of solid thermoplastic resin, particularly when said solid portions are situated immediately adjacent to one or more sections exhibiting cellular structure. Because a solid mass of thermoplastic material will generally take a relatively long period of time to completely cool, warping and stress cracking problems can often develop in such parts. In the case of a molded panel or other item having a solid section of thermoplastic material situated immediately adjacent to one or more sections exhibiting cellular structure, a significant difference in mass exists between said solid section and the cell walls of the adjacent cellular structure. As such, a significant cooling differential can exist between said solid sections and said cell walls, which can in turn lead to significant warpage and stress cracking problems in such a molded panel.
Accordingly, there is a need for a method for manufacturing panels from thermoplastic resins and other moldable materials wherein said panels exhibit a facing sheet and roughly perpendicular cell forming walls extending from said facing sheet in order to define one or more open-faced cells, and wherein said cell forming walls are integrally formed with said facing sheet. Moreover, there is a need for a method of molding panels from thermoplastic resins and other moldable materials which yields uniform and consistent cell forming walls, requires significantly less energy than currently available molding methods, and which further permits molding of panels having one or more sections exhibiting characteristics of a solid panel, such as continuous outer surfaces.
In accordance with the present invention, a molding apparatus and a method for manufacturing panels from thermoplastic resins or other moldable materials is provided. Although the apparatus and method described herein can be used to manufacture panels of many different sizes and shapes, it must be noted that the present invention permits manufacture of both small and large-scale panels. Further, the method of manufacturing panels described herein requires significantly less energy than other known molding methods, and permits molding of panels which are significantly larger than panels which can be molded under other known molding methods.
Furthermore, in accordance with the present invention, there is provided herein a method of manufacturing panels having one or more areas of reduced material. Said areas of reduced material are generally in the form of cellular structure comprising a facing sheet and cell forming walls extending in roughly perpendicular fashion from said facing sheet, and wherein said cell forming walls are integral with said facing sheet. Although said cell forming walls, and thus said cellular structure itself, can be configured in any number of shapes and sizes, the cellular structure is preferably in the form of a hexagonal honeycomb structure to maximize strength characteristics of a panel.
It is frequently advantageous for molded panels to include, in addition to one or more areas defining cellular structure, one or more areas having continuous (that is, non-cellular) outer surfaces. The method of manufacturing panels described herein permits the molding of said panels from thermoplastic resins or other moldable materials with portions thereof alternatively exhibiting cellular structure, or continuous outer surfaces. Further, the method of manufacturing described herein allows for great flexibility with respect to the configuration of said cellular structure and continuous outer surfaces, while alleviating warping and stress cracking problems commonly associated with conventional molding methods.
In contrast to the prior art, in which shallow pans are filled with thermoplastic resin or other moldable material, either in liquid or solid form, and a positive upper mold is used to displace such resin to form cell walls, the present invention utilizes a lower negative mold member which is initially filled with resin or other moldable material. The lower negative mold has a plurality of upwardly facing positive standoffs defining interstitial channels into which thermoplastic resin or other moldable material is filled. Said interstitial channels can be used to define any number of cellular structures; however, in the preferred embodiment said channels form an interconnected honeycomb pattern. Once the lower negative mold has been charged with resin as desired, a generally planar upper mold member is placed on the lower mold. The combined mold assembly is then subjected to elevated heat and pressure, allowing said thermoplastic resin or other moldable material to melt within the interstitial spaces of said lower mold member. After cooling, upper and lower mold members are separated, and a molded panel structure is removed therefrom.
Generally, it is beneficial to manufacture panels having cellular structure, as well as a facing sheet integrally attached to the cell forming walls of said cellular structure. Such a panel is formed by filling the interstitial spaces or channels between standoffs of a lower mold member with thermoplastic resin or other moldable material. Thereafter, a topping layer of such material is placed upon the upper surface of said standoffs, as well as over said channels of the lower mold member which are already filled with such resin or other moldable material. A generally planar upper mold member is then placed on said lower mold member. In order to accommodate a topping layer of thermoplastic resin or other moldable material, the lower mold member is equipped with raised outer containing walls along the peripheral edges of said lower mold member. Further, the positive standoffs of the lower mold member are sized such that the upper surface of said positive standoffs are recessed lower than said outer containing walls of said lower mold member. After melting, said topping layer forms a roughly continuous facing sheet which is integrally attached to said cell walls, which are themselves formed by the thermoplastic resin or other moldable material disposed within the interstitial spaces of said lower mold member.
Additionally, the method of molding disclosed herein permits the inclusion of additives into said panels during the molding process. Because a facing sheet defining one side of a panel can be used as a support surface, it is generally beneficial to increase the frictional characteristics associated with said facing sheet. One way to improve such frictional characteristics is to utilize a type of resin or other moldable material which exhibits different properties than the base resin or other material used to construct the remainder of the panel. Such an additive is ideally loaded into a lower mold member over a resin topping layer which is used to form a facing sheet for a panel.
Other additives, besides those which are intended to improve purely frictional characteristics of the panels, can also be included in panels manufactured pursuant to the method disclosed herein. By way of illustration, materials which are used to disperse electrical charges, such as any member of commercially available anti-static additives, can be loaded into the lower mold member with resin or other moldable material and molded into the panels described herein. Moreover, the additions can also be compounded with thermoplastic resin or other moldable materials prior to the actual molding process. By introducing additives into panels through the disclosed molding process, said additives can be widely and evenly disbursed throughout said molded panels. Alternatively, it may be desirable to load such resins and/or additives in stages, thereby resulting in layering of such resins and/or additives which can greatly improve characteristics of molded parts.
Panels such as those described herein are often used as individual load bearing elements, or to construct larger load bearing structures or surfaces. As such, it is often advantageous to mold such panels from materials having desired strength and durability characteristics. In the case of thermoplastic resins, the resins which exhibit such desired strength and durability characteristics generally have higher densities than other similar resins. When melted, such high density resins are typically very viscous. It is generally thought that structures such as the large-size panels of the type discussed herein cannot be manufactured economically and in large quantities through a compression molding process, because extremely large amounts of energy are required to uniformly distribute such high viscosity resins between positive standoffs of a mold. However, the design of the present invention permits uniform distribution of such resin within interstitial spaces between positive standoffs of a lower mold member without requiring excessive amounts of energy.
The method of manufacture described herein can be utilized to manufacture panels, or specific portions thereof, exhibiting roughly continuous outer surfaces. Such panel portions have roughly the same characteristics as solid sections. However, as discussed above, it is often not feasible to mold such solid sections from thermoplastic resins or other moldable materials due to warping and stress cracking problems associated with differential cooling rates.
In order to mold panels or portions thereof exhibiting roughly continuous outer surfaces, a sacrificial component having open faced cellular structure and an integral facing sheet or skin is first prepared, preferably pursuant to the molding method disclosed herein. Although said sacrificial component can be molded into a particular shape or configuration as desired, it is also possible to simply cut down a portion of a larger previously molded panel having cellular structure to the desired dimensions. The appropriately sized sacrificial component is placed within a pan-like or hollowed out portion of the lower mold member, and covered with thermoplastic resin or other moldable material. The size and shape of the pan-like or hollowed out portion of the lower mold member will depend on the configuration of the finished panel which is desired; for example, if an entire panel exhibiting continuous outer surfaces is desired, then the entire lower mold member would be hollowed out. Conversely, if a panel having both areas of continuous outer surfaces and open faced cellular structure is desired, then only the portions of the lower mold member which correspond to the generally solid portions of the finished panel would be pan-like or hollowed out. Thereafter, an upper mold member of the present invention is installed, and heat and pressure are applied. The thermoplastic resin or other moldable material encases said sacrificial component, thereby resulting in a molded panel, or portion thereof, having roughly continuous outer surfaces. In the preferred method, the sacrificial component is placed within the lower mold member with the open cellular structure of said sacrificial component facing upward. As a result, loading of thermoplastic resin or other moldable material into the lower mold member will result in such resin or other material filling such open cellular structure of said sacrificial component.
It is possible that thermoplastic resins or other moldable materials can be introduced into the lower mold member of the present invention in a molten state. However, for convenience and ease of handling, it is envisioned that said resins or other materials are conveniently added to said lower mold member in solid form as pellets, granules or the like. While said pellets or granules can be of different sizes, it is generally envisioned that pellets of relatively constant size and shape are utilized to fill said lower mold member to facilitate uniform distribution therein. Said pellets must be sized to permit uniform loading of said lower mold member, in general, and the interstitial spaces between positive standoffs of the lower mold member, in particular.
Although the upper and lower mold members can be constructed of any number of functional materials, said mold members are ideally fabricated from metal such as aluminum. Because of the configuration of said upper and lower mold members disclosed herein, significantly more material is required for a lower mold member than an upper mold member of the present invention, which results in a significant differential in mass between said mold members. Such a mass differential typically in turn results in a variance in thermal transfer between said upper and lower mold members during the molding process. Such problems can include different growth and shrinkage rates when said mold members are exposed to elevated temperatures and thereafter allowed to cool. In order to alleviate such problems associated with thermal transfer differentials, spacers can be affixed to said upper mold member. Said spacers serve to reduce the surface area in contact between said upper mold member and a heat source, such as a platen or hot press, which is used to elevate the temperature of the mold members. In the preferred embodiment, said spacers can be of different sizes and shapes to facilitate flexibility in configuration of said spacers on said upper mold member.
The drawings and descriptions of the preferred embodiments set forth herein describe only preferred embodiments of the present invention, simply by way of illustration of the best mode contemplated of carrying out this invention. It must be noted that this invention is capable of other and different embodiments, and its several details are capable of modification in various respects, all without departing from the invention.